The present invention relates to optical calibration and, more particularly, to a standard substance for calibration of an optical measurement device for measuring optical transmission and scattering.
Optical measurement devices exist for analyzing a sample of a material based on the transmission or scattering of light passing therethrough. For example, a device is known which measures the coagulation time of directing a light through a mixture of blood plasma and a sample chemical. Coagulation is detected by a change in the light-scattering property of the mixture due to increased turbidity when fibrinogen in the blood plasma is converted to fibrin during coagulation.
Such optical measurement devices require calibration to maintain their accuracy under varying conditions over time. One way of calibrating such devices employs a standard substance having known properties of light transmission and scattering. One possible standard substance includes a sample of the material which is to be measured. In the case of the coagulation-time measurement apparatus described above, the standard substance is a mixture of blood plasma and chemical which is to be measured. This has not proven to be a successful technique since such a sample lacks long-term stability and requires frequent adjustment of the measurement device.
Other standard substances include, for example, agar, gelatin, acrylamide gel, chitin and geldium jelly. The turbidity of the above substances can be adjusted by changing the concentration of the ingredients. The turbidity of agar can be changed by changing the concentration of agarose, which is one of its main ingredients. However, since agar, as well as the remainder of the above candidate standard substances, contain a high concentration of water, care must be taken to avoid evaporation of the water, and the resulting change in turbidity. This generally requires a closely plugged container.
In addition, the above standard substances are gels. As is well known, the lattice structure of most gels is unstable and has no resistance to shaking. Thus, long-term stability is difficult to maintain.
A further problem with the above prior substances is that they contain organic substances which are nutritious to bacteria. For example, agarose is an organic substance that is hospitable to bacteria. Bacteria contamination is a further threat to long-term stability.
Because of the above problems with organic gels, it appears desirable to employ an inorganic standard substance for calibration of an optical detector.
One type of inorganic standard substance includes a glass standard scattering substance such as, for example, that disclosed in Japanese Examined Patent Publication No. 57-41688, published in 1982. This material is made by mixing a small quantity of impurities to silicon oxide. The mixture is melted and baked at a suitable temperature. As a result of the melting and baking, the impurities form small nuclear crystals. Thus is formed a glass in a boric acid phase. The resulting glass scattering substance is placed in a transparent container. Gaps in the container are filled with a liquid.
This technique has the disadvantage that the process for making the glass standard is complicated, and the resulting device is not completely stable. Several attempts are typically required to determine the amount of turbidity-enhancing substance to add. The resulting glass must be melted and formed to conform to the shape of the light-measuring device. Consequently, the device is generally larger than desired and more costly. Also, since the device is stored in a container, it is difficult to maintain the scattering substance stably in the container. Hence its optical stability is also difficult to maintain.
Alternatively, the glass standard substance can be formed as a hollow glass tube. The amount of scattering can be varied by changing the inner diameter of the tube. The amount of scattering is increased by making the inner diameter smaller, and is decreased by making the inner diameter larger.
The optical properties of this device are degraded if any portion of the outer or inner surface is deformed. In addition, such a glass tube tends to shrink over time, with resulting optical instability.